13 
differences with respect to a mean horizontal datum plane do not surpass 
+ 3 metres), photo maps at 1:5,000 may be produced by conventional recti- 
fication. For this a Rectifier is needed. The cost of such an instrument is 
about 2/3 that of a precision plotter. For the production of photo maps at 
1:5,000 by conventional rectification of 1:11,500 photography, covering an 
area of 5000 square kilometres annually, only one Rectifier is necessary. 
1.5.4 Aerial Triangulation 
For photo mapping at 1:5,000, in our example, from 1:11,500 photo- 
graphy, the required control can be determined most economically by aerial 
triangulation of the 1:11,500 photographs. Together with the ground 
co-ordinates of the minor control points, preferably also those of a grid of 
reference points (sharply identifiable permanent objects, or pre-signalised 
monumented points) should be determined. The reference points are needed 
for field completion and field revision surveys. The intervals between the 
reference points should be chosen according to the methods and equipment 
envisaged for the field completion and revision. The intervals may range 
from 200 to 2000 metres. 
For high precision photogrammetric stereo-restitution for the recon- 
struction of boundaries, aerial triangulation and subsequent block adjustment 
do not affect the high relative accuracy within each stereo model, and even 
improve the relative accuracy of neighbouring models, as compared to the 
use of full ground control for each stereo model. With planimetric ground 
control at appropriate spacings along the perimeter of the block only (and 
sufficient ground heights so that the minor control points have sufficient 
height accuracy for proper levelling of the individual models), a very high 
absolute planimetric accuracy is obtainable. 
1.5.5 Annotated Photo-Maps 
Rectification—either conventional rectification in the case of flat 
terrain, or differential rectification in the case of non-flat terrain—is usually 
employed using diapositives (on glass or film) in the rectifying instrument, 
so that the first product is usually an orthophoto negative, normally on film. 
Next these orhtophoto negatives must be assembled to form photomap 
sheets. It would be ideal if the photo flight (see 1.5.7 below) were carried